The present invention relates in general to the detection of the positions of articles transported on a conveyor. The invention has been developed with particular attention to the detection of the relative positions of articles transported on a conveyor in an automatic packaging plant. It may, however, also be applied in other fields.
In general, when there is a continuous or substantially continuous flow of articles on a conveyor, the need may arise to detect, in one or more positions on the conveyor path, whether the advancing articles are spaced apart or in contact with each other or almost in contact, that is to say, in a condition in which a certain accumulation or queue has formed or is tending to form.
In order to ensure that the plant operates well, it is essential to detect this condition accurately, for example, to prevent the creation of excessive longitudinal pressure in the advancing line of articles.
In many cases (and particularly in the typical situations of application to which reference will be made below, that is, in the automatic packaging of food products such as chocolate bars, chocolate-coated wafer biscuits and like products) the articles in question tend to be rather delicate whereby longitudinal pressure in the line of articles would cause damage to the products themselves.
In a conventional solution in the automatic packaging and transportation industry, the articles in question are advanced in a continuous or substantially continuous flow on conveyors such as endless belts each of which has an upper pass for transporting the articles and a lower return pass.
In this connection, the use of optical gates or barriers to detect the relative positions of the articles (that is, whether they are advancing spaced apart or in contact with each other or almost in contact) is known.
It is known to use various operative solutions for this purpose, amongst these may be mentioned:
optical gates each constituted by a light emitter (an LED or the like) and a photoelectric receiver (a photodiode or a phototransistor) situated on opposite sides of the flow of products so that the beam of light travels from the emitter to the receiver in a direction generally transverse (perpendicular or oblique to) the direction of advance of the articles and is intercepted periodically by the passage of the articles so that a permanent or almost permanent interruption of the beam is indicative of the condition in which they are in contact or almost in contact; PA1 optical gates in which the emitter and the receiver are situated on one side only of the flow of products whilst a reflective screen is situated on the opposite side for returning the radiation produced by the emitter to the receiver: again in this case, the transit of the articles obstructs the path of propagation of the light beam and results in a disturbance of the signal output by the receiver which can be used to detect whether the products are spaced-apart or are packed closely together or near to each other, and PA1 solutions which provide for the use of an emitter-receiver pair situated on one side of the flow of articles with an associated optical device whose focus is adjusted so as to coincide approximately with the surface of the advancing articles; in this case, a reflective element is not provided on the opposite side of the flow: the detection by the photodetector is in this case based on the different levels assumed by the photodetection signal according to whether the radiation generated by the emitter is reflected (or, more correctly, scattered back) by the surfaces of the advancing articles in correspondence with the focus of the optical device or by the background which is generally constituted by the surface of the conveyor or by the region behind it.
All three of the solutions described above have advantages and disadvantages.
The first two solutions (the emitter-detector situated on opposite sides or the emitter-detector on one side with a reflective element on the opposite side) are very well suited to horizontal detection, that is, with the path of propagation of the radiation of the light gate extending in a plane parallel to, or almost parallel to, the plane on which the products advance.
In the case of products advancing between lateral containment guides, however, it is necessary to provide apertures for the passage of the light.
In the case of articles having a certain height, this does not create any particular problem since the guides can be formed so as to leave a horizontal strip open between their lower edges and the surface of the conveyor so that the optical gate effects its detection in correspondence with the bases of the products. If the products are not very high, however, (one thinks, for example, of chocolate-coated wafer biscuits) it is necessary to provide windows in the lateral containment guides; but these windows may cause the products to jam, with very harmful results.
In any case, regardless of whether or not the guides are present, lateral or horizontal detection is very difficult with articles which are not very tall since even very small vertical movements may distort the results of the detection quite significantly.
The use of the third solution (an emitter-detector unit with optical focusing devices) resolves the problems described above by detecting vertically, that is, with the detector unit situated above the flow of articles advancing on the conveyor. This solution suffers, however, from the fact that the reflection (or backscattering) of the optical radiation on which its operation is based is particularly critical.
In particular, and above all in application to food products, the reflection or backscattering characteristics of the surfaces of the articles transported on the conveyor may differ substantially both in absolute terms and from one article to another.
Still with reference to the case of chocolate-coated wafer biscuits, it can readily be understood that the surface of the chocolate coating is not at all flat and uniform and usually has unpredictable ripples and wrinkles. Moreover, the Applicant has found that even small changes in the nature of the coating (a change from sweet chocolate to bitter chocolate) can cause very marked changes in the backscattering characteristics of the surface of the product.
The foregoing also applies essentially to backscattering by the surrounding area or the surface of the conveyor.
In some situations, for example, backscattering by an internal surface of the housing of the machine or by an object placed accidentally on the ground beneath the machine, even at a certain distance from the optical unit, may be confused with backscattering from the surface of the product in the focal plane of the optical device.
The same is also true of the surface of the conveyor. In this respect, it should also be remembered that many food products are quite dark in colour and have quite uneven surfaces (which thus tend not to be very reflective) whilst the current standards for the manufacture of machines and conveyors generally dictate the use of light colours (typically white) or materials (for example plastics materials for the food industry) which are usually quite smooth and reflective.
This situation is further complicated by the fact that the conveyor surface may accidentally become soiled by traces of the product (for example smears of chocolate) which can further alter the backscattering characteristics of the support.
The object of the present invention is therefore to provide a method and apparatus for detecting the positions of articles transported on a conveyor which avoids the problems of the known solutions referred to above.
This object is achieved according to the present invention by positioning the radiation emitter and detector units along an axis which is perpendicular to the plane of the conveyor belt, and by use of a detector unit which includes a pair of detectors which are offset from each other in the direction of travel of the conveyor belt.
Other objects, advantages and novel features of the present invention will become apparent form the following detailed description of the invention when considered in conjunction with the accompanying drawings.